The present invention relates to enhanced cooling of generator rotors.
The power output rating of dynamoelectric machines, such as large turbo-generators, is often limited by the ability to provide additional current through the rotor field winding because of temperature limitations imposed on the electrical conductor insulation. Therefore, effective cooling of the rotor winding contributes directly to the output capability of the machine. This is especially true of the rotor end region, where direct, forced cooling is difficult and expensive due to the, typical construction of these machines. As prevailing market trends require higher efficiency and higher reliability in lower cost, higher-power density generators, cooling the rotor end region becomes a limiting factor.
Turbo-generator rotors typically consist of concentric rectangular coils mounted in slots in a rotor. The end portions of the coils (commonly referred to as endwindings), which are beyond the support of the main rotor body, are typically supported against rotational forces by a retaining ring (see FIG. 1). Support blocks are placed intermittently between the concentric coil endwindings to maintain relative position and to add mechanical stability for axial loads, such as thermal loads (see FIG. 2). Additionally, the copper coils are constrained radially by the retaining ring on their outer radius, which counteracts centrifugal forces. The presence of the spaceblocks and retaining ring results in a number of coolant regions exposed to the copper coils. The primary coolant path is axial, between the spindle and the bottom of the endwindings. Also, discrete cavities are formed between coils by the bounding surfaces of the coils, blocks and the inner surface of the retaining ring structure. The endwindings are exposed to coolant that is driven by rotational forces from radially below the endwindings into these cavities (see FIG. 3). This heat transfer tends to be low. This is because according to computed flow pathlines in a single rotating end winding cavity from a computational fluid dynamic analysis, the coolant flow enters the cavity, traverses through a primary circulation and exits the cavity. Typically, the circulation results in low heat transfer coefficients especially near the center of the cavity. Thus, while this is a means for heat removal in the endwindings, it is relatively inefficient.
Various schemes have been used to route additional cooling gas through the rotor end region. All of these cooling schemes rely on either (1) making cooling passages directly in the copper conductors by machining grooves or forming channels in the conductors, and then pumping the gas to some other region of the machine, and/or (2) creating regions of relatively higher and lower pressures with the addition of baffles, flow channels and pumping elements to force the cooling gas to pass over the conductor surfaces.
Some systems penetrate the highly stressed rotor retaining ring with radial holes to allow cooling gas to be pumped directly alongside the rotor endwindings and discharged into the air gap, although such systems can have only limited usefulness due to the high mechanical stress and fatigue life considerations relating to the retaining ring.
If the conventional forced rotor end cooling schemes are used, considerable complexity and cost are added to rotor construction. For example, directly cooled conductors must be machined or fabricated to form the cooling passages. In addition, an exit manifold must be provided to discharge the gas somewhere in the rotor. The forced cooling schemes require the rotor end region to be divided into separate pressure zones, with the addition of numerous baffles, flow channels and pumping elementsxe2x80x94which again add complexity and cost.
If none of these forced or direct cooling schemes are used, then the rotor endwindings are cooled passively. Passive cooling relies on the centrifugal and rotational forces of the rotor to circulate gas in the blind, dead-end cavities formed between concentric rotor windings. Passive cooling of rotor endwindings is sometimes also called xe2x80x9cfree convectionxe2x80x9d cooling.
Passive cooling provides the advantage of minimum complexity and cost, although heat removal capability is diminished when compared with the active systems of direct and forced cooling. Any cooling gas entering the cavities between concentric rotor windings must exit through the same opening since these cavities are otherwise enclosedxe2x80x94the four xe2x80x9cside wallsxe2x80x9d of a typical cavity are formed by the concentric conductors and the insulating blocks that separate them, with the xe2x80x9cbottomxe2x80x9d (radially outward) wall formed by the retaining ring that supports the endwindings against rotation. Cooling gas enters from the annular space between the conductors and the rotor spindle. Heat removal is thus limited by the low circulation velocity of the gas in the cavity and the limited amount of the gas that can enter and leave these spaces.
In typical configurations, the cooling gas in the end region has not yet been fully accelerated to rotor speed, that is, the cooling gas is rotating at part rotor speed. As the fluid is driven into a cavity by means of the relative velocity impact between the rotor and the fluid, the heat transfer coefficient is typically highest near the spaceblock that is downstream relative to the flow directionxe2x80x94where the fluid enters with high momentum and where the fluid coolant is coldest. The heat transfer coefficient is also typically high around the cavity periphery. The center of the cavity receives the least cooling.
Increasing the heat removal capability of passive cooling systems will increase the current carrying capability of the rotor providing increased rating capability of the generator whole maintaining the advantage of low cost, simple and reliable construction.
U.S. Pat. No. 5,644,179, the disclosure of which is incorporated by reference describes a method for augmenting heat transfer by increasing the flow velocity of the large single flow circulation cell by introducing additional cooling flow directly into, and in the same direction as, the naturally occurring flow cell. This is shown in FIGS. 4 and 5. While this method increases the heat transfer in the cavity by augmenting the strength of the circulation cell, the center region of the rotor cavity was still left with low velocity and therefore low heat transfer. The same low heat transfer still persists in the corner regions.
Spaceblocks are an essential feature of generator endwindings. In addition to defining the cavity spaces between concentric rotor coils where the cooling of the endwinding occurs, the spaceblocks enhance the cooling flow entrained into the cavity. The process of entraining cooling flow into the rotating cavities, can be viewed as made up of two factors: A radially outward centrifugal force and a radial return path enhanced by a low-pressure region.
The present invention provides for enhanced cooling of generator rotors. More particularly, in an embodiment of the invention, a re-entrant geometric feature is provided on the trailing or downstream wall of the spaceblock to cause a further pressure reduction in the low-pressure region defined adjacent the downstream wall of the spaceblock. The ability of the spaceblock to enhance the radially outward centrifugal force that entrains cooling flow into the rotating cavity is not attenuated by the re-entrant geometric feature.
More specifically, the invention proposes to enhance the rotating cavity cooling flow by lowering the suction pressure developed at the trailing edge of the spaceblock. In an embodiment of the invention, this effect is produced by the introduction of a re-entrant contour to the existing block configuration while maintaining the block""s ability to aid the entrainment of cooling flow from the annular passage, between the rotor coils and the shaft spindle, into the cavity.
In an exemplary embodiment, the invention modifies existing spaceblocks thus minimizing impact on the overall design of the generator assembly.
Thus, the invention is embodied in a gas cooled dynamoelectric machine that comprises a rotor having a body portion, axially extending coils, end turns defining a plurality of endwindings and at least one spaceblock is located between adjacent endwindings so as to define cavities therebetween, and wherein a downstream wall of the spaceblock has a non-planar contour to lower a suction pressure developed adjacent the trailing edge of the spaceblock to enhance cooling flow.